Field
The present invention relates to an optical imaging system.
Description of the Related Art
In an optical imaging system for detecting signal lights with a specific wavelength, a bandpass filter adapted to the specific wavelength is used. By means of the bandpass filter, noises can be removed without degradation of the signal lights.
Conventionally, in such an optical imaging system, the bandpass filter is located near the image surface because of facilitation of assembly or the like (for example, FIG. 1 of WO2010/103595A1).
On the other hand, in such an optical imaging system, when the F-number is reduced, the numerical aperture on the image side becomes greater. As a result, an angle of incidence onto a surface of the filter located near the image surface, of a marginal ray of a light beam which enters the optical imaging system and the principal ray of which is parallel to the optical axis becomes greater. Accordingly, the transmittance of the filter decreases, and that causes a problem that a sufficient amount of light does not reach a sensor located on the image surface even if the F-number is reduced in the optical imaging system.
Thus, an optical imaging system which is configured such that a sufficient amount of light reaches a sensor located on the image surface even when the optical imaging system has a reduced F-number and is provided with the filter has not been developed.
Accordingly, there is a need for an optical imaging system which is configured such that a sufficient amount of light reaches a sensor located on the image surface even when the optical imaging system has a reduced F-number and is provided with the filter.